Mitochondria are dynamic organelles that maintain the health of myelinated axons by metabolic matching, in which mitochondria move to and pause at regions of high energy demands. In demyelinating diseases such as multiple sclerosis (MS), there is a huge increase in energy demand as large areas of denuded axons are exposed by the loss of myelin. In response, more mitochondria move to and become immobilized at lesion sites, contributing to a large increase in axonal mitochondrial content. The prevailing hypothesis is that this increase in axonal mitochondria content is an adaptive change to maintain proper metabolism to protect axons. However, this mitochondrial increase could also be maladaptive and tip the scale to damage axons by excess free radical production. Which way the scale actually tips has never been tested. With mitochondria emerging as a key player in the pathogenesis of MS, it becomes critical to determine whether this well-known increase in axonal mitochondrial content is friend or foe. Recently we started to address this issue by genetically reducing the increase in axonal mitochondrial content in an animal model of dysmyelination (Shiverer). We interbred the Shiverer into a background lacking a mitochondrial immobilization protein (syntaphilin) that normally contributes to the increase in axonal mitochondrial content. Surprisingly, deletion of syntaphilin dramatically extends the life expectancy of the Shiverer mutant. Based on this survival data, this R21 explores whether mitochondrial anchoring contributes to axon killing in demyelination, and whether increasing mitochondrial mobility by elimination of immobilization confers neuroprotection on demyelinated axons. In Aim #1, we will use immunohistochemistry and morphologic analysis to see if axons are protected in Shiverer mice lacking syntaphilin. In Shiverer (with syntaphilin present) there is an age-dependent axonal degeneration in various regions in the CNS. We will examine if elimination of mitochondrial anchoring delays or reduces this age-dependent axonal degeneration. We will also transfect syntaphilin in the DRG to directly test if excessive mitochondrial anchoring kills axons in the spinal cord. In Aim #2, we will examine if elimination of mitochondrial anchoring similarly protects mice in EAE, an inflammatory demyelination model for MS. We will induce EAE in mice with or without syntaphilin to see if the clinical scores are improved in the syntaphilin-null background. Conclusion: This R21 proposal explores a potentially ground-breaking paradigm in mitochondrial research in MS. In contrast to existing research focusing on mitochondria as a down-stream target of upstream degenerating changes, we might have pinpointed an upstream event involving mitochondrial dynamics (excessive anchoring) that leads to downstream degeneration. It further suggests an exciting therapeutic possibility that altering the ratio of mobile to immobile mitochondria could profoundly alter disease progression. The proposed experiments will determine whether there is firm footing for a future R01 application.